Antennas for handheld portable equipment, for example pagers, portable telephones and cellular telephones, must be small in size, light in weight, and compact in physical volume. Flush-mounted or built-in internal antennas are often required, and PIFAs are particularly attractive for applications of this type. For many installations, a PIFA is a preferred choice for use as an internal antenna in cellular communications applications.
PIFAs are so named because from a side view a PIFA having an air dielectric resembles the letter F with its face down (see for example section 10.7 of the publication MICROSTRIP ANTENNA DESIGN HANDBOOK by R. Garg, P. Bhartia, I. Bahl and A. Ittipiboon, Copyright 2001 Artech House, Inc.).
PIFA technological and design progress has led to size-miniaturization, and to the enhancement of the multi band performance of a single feed PIFA. The multi band performance capability of a single feed PIFA has also been advanced to simultaneously include both dual cellular and dual non-cellular applications.
PIFA designs can include the formation of a slot in the PIFA's radiating element. For example, U.S. Pat. No. 6,573,869, incorporated herein by reference, provides a multi-band PIFA having a radiator with a spiral slot formed therein to cause multiple frequency dependent nulls in the antenna's electric field modal distribution.
Choices of the position, contour and the length of a slot within a PIFA's radiating element depend on the design parameters of interest, and at times more than one slot is preferred within the PIFA's radiating element.
Using a slot to physical partition the radiating element of a single band PIFA for multi band operation, as well as providing a slot as a reactive loading tool to reduce the resonant frequencies of the radiating element, form two important functional roles of a slot in a PIFA's radiating element. In addition, the position and the contour of the slot can be chosen to control the polarization characteristics of the upper resonant band of a multi band PIFA.
The introduction of a slot within a PIFA's radiating element has the undesirable effect of reducing the effective surface area of the radiating element, which in turn leads to a degrading effect on both the gain and the bandwidth of the PIFA.
In addition to having a slot, the radiating element of a PIFA can also be associated with capacitive loading elements, usually in the form of bent metal segments or tabs at the edges of the radiating element, these segments extending downward toward the ground plane without touching the ground plane. However, capacitive loading has a negative impact on both the bandwidth and the gain of the PIFA. As an example, U.S. Pat. No. 5,764,190, incorporated herein by reference, provides a capacitive loaded PIFA.
Slot loading and capacitive loading are most often used to achieve a desired resonance without increasing the physical size of the PIFA.
Despite the above-mentioned shortcoming of a slot on the performance of a PIFA, the formation of a slot within the PIFA's radiating element may be necessary in order to realize multi band performance, as well as to provide a desired resonance.
Likewise, capacitive loading can be a common requirement in PIFA designs in view of severe constraints that are placed on the physical volume that is available for placement of an internal antenna within a wireless device.
As an example of the use of a meandering pattern within the radiating stricture of a PIFA, U.S. Pat. No. 6,380,895, incorporated herein by reference, provides a radiating structure for a microstrip PIFA wherein a first patch s connected to a second patch by way of a meandering pattern. The first patch comprises means for feeding an RF signal to the radiating structure, and the meandering pattern acts as an inductive connection between the two patches.